In an optical disc device for recording information on an optical disc or reproducing information recorded on an optical disc, an optical servo for maintaining the focus of a laser beam onto an information recording track has been requiring to improve precision thereof at a rapid rate along with increase in the speed and density growth of optical discs, and attentions have been drawn to repetition control as a means to improve the precision.
FIG. 10 is a diagram illustrating a construction of a repetition control circuit in a conventional optical disc device. In the figure, reference numeral 101 denotes an adder for adding a compensated signal 100 having a periodical component, such as an error signal which is to become a tracking object, and an output signal of a circumference memory, i.e., a learning memory 104 which will be described below. Reference numeral 102 denotes an attenuation gain β for varying the degree of learning.
Reference numeral 103 denotes a low-pass filter for passing lower components of output signal of the learning memory 104, reference numeral 104 denotes a learning memory for storing frequency components of information corresponding to a single period (single rotation) of an optical disc, reference numerals 105 and 106 denote gain elements for switching the compensated signals to be stored in the learning memory 104, reference numeral 107 denotes an adder for adding output signals of the gain elements 105, 106, reference numeral 108 denotes a subtractor for subtracting the output signal of the low-pass filer 111 from the compensated signal 100, reference numeral 109 denotes an absolute value detection unit for detecting the absolute value of an output signal of the subtractor 100, reference numeral 110 denotes a comparator for comparing the output signal of the absolute value detection unit 109 with a predetermined value, and reference numeral 111 denotes a low-pass filter for passing lower components of the compensated signal 100.
By having the construction as described above, the compensated signal 100 having a periodic component, which is to become an input signal is first determined by a correlation detection unit 112 constituted by the low-pass filter 111, the subtractor 108, the absolute value detector 109, and the comparator 110 whether it has periodicity or not.
This is carried out to detect whether or not the compensated signal 100 which is inputted thereto is superposed with noise, disturbances, and influences due to scratches on the optical disc. And signal comparison is carried out at the correlation detection unit 112, and when there is a correlation, i.e., when noise is not detected as a result of the comparison, k=1 is outputted and signals corresponding to a single rotation of the disc are stored in the memory 104 via the gain element 106.
The signals stored in the memory 104 are fed back via the low-pass filter 103 and a gain element 102 for varying the degree of learning so as to satisfy stability conditions for repetition control.
Further, when it is detected by the correlation detection unit 122 that there is no correlation in a result of the signal comparison, i.e., when noise is detected, K=0 is outputted and storing of new signals is stopped by the gain element 106.
At this time, if the signals corresponding to a single rotation of the optical disc that are stored immediately before are stored in the memory again by the gain element 105, the tracking capability can be improved without attenuating the degree of learning even when disturbances are mixed.
Patent Document 1: Japanese Published Patent Application No. Hei. 9-50303